6.1.3 (a,b) Properties of Carboxylic Acids

(a) explanation of the water-solubility of Carboxylic Acids in terms of Hydrogen bonding

(b) reactions in aqueous conditions of Carboxylic Acids with Metals and Bases (including Carbonates, Metal Oxides and Alkalis)

{Comparison of acidity of different Carboxylic Acids not required.}

Solubility

So, a short chain carboxylic acid will probably dissolve well in water because a large proportion of the molecule has similar intermolecular forces.

But as the length of the Alkyl group increases, we are increasing the hydrophobic part of the molecule without increasing the strength or number of Hydrogen bonds.

Eventually, so much of the molecule is hydrophobic compared to the hydrophilic COOH group that the molecule stops being soluble.

You are not required to know exactly how long the chain should be for this to happen but you should be able to discuss the general trend and be able to draw a diagram of Hydrogen bonding from an acid molecule to a water molecule.

You should recall from Year 9 that:

Acid + Alkali/Base --> Salt + Water

Acid + Carbonate --> Salt + Water + Carbon Dioxide

{ Acid + Reactive Metal --> Salt + Hydrogen }

Carboxylic acids are not very strong - they only partly dissociate CH₃COOH ⇌ CH₃COO^- + H⁺

Remember that this means that, for instance, 1 mole of Ethanoic Acid reacts with exactly as much Sodium Hydroxide as 1 mole of HCl (because they both produce 1 mole of H⁺ ions) but more slowly because in HCl all the Hydrogen ions are available immediately since it fully dissociates.

Unlike Phenols, Carboxylic acids are strong enough to react with weak bases like Carbonate and so whether a weakly acidic organic substance fizzes in Sodium Carbonate solution is a useful way of distinguishing between them.

There is no need to treat Carboxylic acids as any different to a bench acid (like HCl) when writing equations:

• With Sodium Hydroxide:

HCl: HCl + NaOH --> NaCl + H₂O

Essentially, the Na⁺ ion replaced the acid's H⁺ ion.

Doing the same with Ethanoic acid gives us...

CH₃COOH + NaOH --> CH₃COONa + H₂O

• With Sodium Oxide:

HCl: 2 HCl + Na₂O -->2 NaCl + H₂O

Again, the Na⁺ ion replaced the acid's H⁺ ion.

Doing the same with Ethanoic acid gives us

2 CH₃COOH + Na₂O --> 2 CH₃COONa + H₂O

• With Sodium Carbonate:

HCl: 2 HCl + Na₂CO₃ -->2 NaCl + H₂O + CO₂

Again, the Na⁺ ion replaced the acid's H⁺ ion.

Doing the same with Ethanoic acid gives us

2 CH₃COOH + Na₂CO₃ --> 2 CH₃COONa + H₂O + CO₂

Strictly speaking, when a metal reacts with an acid we are talking about a redox reaction because the Oxidation number of the Hydrogen drops from +1 in H⁺ to 0 in H₂.

In normal neutralisation reactions, it remains +1 (no change = not REDOX).

Nevertheless, if you use up all the Hydrogen ions in an acid the solution still becomes neutral.

HCl: 2 HCl + Na-->2 NaCl + H₂

Again, the Na⁺ ion replaced the acid's H⁺ ion.

Doing the same with Ethanoic acid gives us

2 CH₃COOH + Na--> 2 CH₃COONa + H₂

Or,

HCl: 2 HCl + Mg--> MgCl₂ + H₂

This time the Mg²⁺ ion replaced the acid's H⁺ ion, but requires two of them because the Cl^- ion is only 1-.

Doing the same with Ethanoic acid gives us

2 CH₃COOH + Mg--> (CH₃COO)₂Mg + H₂